Correct location to install the CW makeup valve on a boiler

hobbit · September 14

Hello all, Steve from the wonderful Catskill mountain park.

I have a question regarding a hot water boilers CW makeup valve location. The valve is currently installed into a 3/4" pipe on the top front of a hot water boiler next to the supply pipe. This pipe is another alternative location for the pressure relief valve which is installed at the recommended top rear location. I read an article that said feeding the CW at this top location directly into a boiler is not ideal and can be causing the excessive air problems. The directions for this boiler shows the PRV installed on the return end above the circulator. Would anyone recommend removing and reinstall the CW feed to the return end at the suction side of the circulator, or is the supply side near the expansion tank a better location?

hot_rod · September 14

The water fill should connect at or near the expansion tank. And ideally the circulator is pumping away from the tank/ fill location. A backflow device on the fill valve also

A common piping example here:

hobbit · September 14

Thanks Hot Rod. This boiler was shipped with the circulator on the return side. The supply side has a Watts air scoop with the expansion tank below and air vent on top. There are two zone valves on the supply side after the expansion tank. The Watts air scoop doesn't have the side tap. I could install a 1/2" T between the scoop and tank and feed the CW makeup there? Would that be better than using the return side or where it's currently feeding through the top of the boiler? I included the boiler and feed valve recommendations as well as the air scoop and tank.

hot_rod · September 14

Yeah, there are many hydronic systems out in the wild with pumps on the return, air purger and expansion on the boiler supply.

Hydronic "best practices" would have the expansion tank located at the inlet of the circulator..

So in your case you would need to either move the circ up after the air purger on the supply

or move the expansion tank connection down before the circulator on the return.

If it works or has worked without any air noise issues you could leave it as is.

An example of the expansion tank moved to return. I'd like to see an air purger added to this drawing also. Float tytpe air vents positioned like shown do not remove air from fluid stream very well.

Your system does have a primitive, ramp type air purger :) Ideally a purger like yours has 12- 18" of straight pipe upstream.

EdTheHeaterMan · September 14

This piping arrangement is very old. It actually causes some problems in some systems. Many installers will use a pressure reducing valve (auto feed) on the CW supply to the boiler.

Imagine in your minds eye the pressure at these 5 locations (A, B, C, D, E) in this illustration. When you fill the system using the auto feed valve set at about 12 PSI. you have a system full of cold water and an expansion tank with 12 PSI air pressure so it has no water in the expansion tank. with the circulator pump off, all the pressure gauges will read close to 12 PSI.

Now what happens when the pump turns on?

There will be a pressure difference on either side of the pump. and if we assign a 6 PSI difference to that pump (your actual pressure difference may be different). Now it is a known fact that the expansion tank with 12 PSI in in is known as the Point Of No Pressure Change (PONPC) so unless you add water or air to the closed system OR you change the water temperature, there will always be 12 PSI at gauge B

Now if we allow for about 0.5 PSI pressure drop thru the boiler, when the circulator pump comes on the pressure at the pump outlet might change to 12.5 PSI on gauge E. That means that the inlet side of the pump will change to 6.5 PSI at gauge D.

What is the pressure at gauge C if the pressure at gauge D is 6.5 PSI? Probably 6.5 PSI. And what is the pressure reducing valve set to? 12 PSI right? Then what will the pressure reducing valve (Auto Feed) do it there is only 6.5 PSI on the side that is supposed to be 12 PSI? Add Water to make the inlet side of the pump 12 PSI, right?

If the pressure at the inlet side of the pump is 12 PSI then you add 6 PSI to the outlet side of the pump that will make it 18 PSI… Right?

That will cause more water to enter the system and that additional water has no place to go because the system is already full of water… so it ends up in the expansion tank at 18 PSI. Now there is less air space in the tank for expansion when the water heats up. so maybe the system will actually get close to 28 or 30 PSI because there is not enough room for the water to expand.

Now you have a boiler that is operating and the temperature increases and you end up with the following pressure gauge readings.

A = 28 PSI
B = 28.5 PSI due to the pressure drop thru the flow control valve (Aprox)
C = 23 PSI due to the increase in pressure as a result on heating the water
D = 23 PSI for the same reason as C
E = 29 PSI as a result of the increase in pressure folr the pump operating against the PONPC and the 0.5 pressure drop thru the boiler. (all hypothetical numbers)

And every once in a while the pressure relief valve weeps a little water. Eventually the relief valve fails to close completely and you have a leaking valve that needs to be replaced and you can't figure out why they keep going bad every other year, while the water heater relief valves lasts over 10 years without. problem.

That old diagram is a relic from back when pump seals were made of heat sensitive fibrous material so all the pumps were placed on the cooler return piping. Move into the the 21st century and place your pump so the expansion tank is on the inlet of the pump.

hot_rod · September 14

Ed is on the correct path

To find exact numbers to the pressures at various points you would need to define the circuit, hydraulically speaking.

How many feet of piping, how many fittings and valves, etc

In some cases you could be sub atmospheric in points of the system. A condition you can and should avoid with proper component placement

Here is a visual of

pumping away, on a hypothetical circuit

Correct location to install the CW makeup valve on a boiler

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